A protection circuit of this general type is already known from DE-OS No. 32 03 829. In this circuit, in order to avoid thermal overloading of the stator winding in the event of the motor stalling, a transistor becomes conductive, the emitter-collector path of which is disposed in series with the stator winding and the commutation transistor connected thereto, and the base of which carries the voltage induced in the stator winding. This circuit is suitable for motors of smaller power capacity. In motors of higher capacity there is a danger of overheating and therefore of destruction of the stalling protection transistor. Also in larger motors there are difficulties at the time of starting. This is attributable to the fact that, because of the required higher starting torque, an RC-circuit provided for automatic starting has to be made considerably larger, and the space available in the motor is insufficient for such a circuit.
Furthermore an electronic overload protection circuit arrangement is known from DE-AS No. 23 61 645. In this arrangement, a resistor is provided in series with the drive winding for receiving the induction current. In parallel with this resistor is a delay circuit which is connected to the output of a transistor switching device consisting of two transistors, one transistor of which is disposed in the supply circuit of the winding, the supply to the circuit being cut off when there is excess current in this transistor. Furthermore a lifting device is provided which is operable from outside and which removes the blocking caused by the excess current in this transistor.
A circuit arrangement for current control of a direct current motor is known from DE-PS No. 31 39 545, in which arrangement the voltage across a measurement resistor is used as the control voltage for a transistor. Driving of this transistor results in discharging of a timing circuit, which leads via an amplifier circuit to an interruption of the motor current and thereby to a lowering of the voltage across the measurement resistor. If the operating voltage of the transistor falls below a certain value, the transistor is turned off and the timing circuit is freed again. In dependence on the induction current in the windings during turning of the motor, the driving of the timing circuit resumes and again brings about current supply to the windings.
The invention has as its object to develop a protection circuit which ensures effective protection of the motor in the case of stalling of the rotor and which can be constructed in a space-saving way.
According to the invention there is provided a protection circuit for stalling protection in commutatorless direct current motors, for example in such motors driven by Hall-generators, in particular Hall-IC's, in which a respective semiconductor circuit element disposed in series with each stator winding is turned on or off in dependence on a control voltage induced in the stator windings, wherein there is used as the control voltage the sum of the operating voltage in the unenergised windings and the counter-e.m.f. induced in these windings, whereby, if the sum of these voltages is greater than the operating voltage, the semiconductor circuit element connected to the respective energised winding is turned on, and, if the counter-e.m.f. is zero or falls below a predetermined cut-off value, the respective semiconductor circuit element is turned off.
In this way cut-out of the motor takes place in dependence on the magnitude of the counter-e.m.f. in the respective unenergised winding, which is advantageous, since the magnitude of the counter-e.m.f. is directly dependent on the rate of rotation of the motor. According to the invention it is thereby possible not only to achieve a cut-out when the motor has stopped but indeed also when the rate of rotation of the motor falls below a minimum rate of rotation.
Furthermore it is advantageous according to the invention, in the starting phase of the direct current motor, if start-up bridging circuit ensures by means of a delay circuit that, for the time interval for which the voltage value of the counter-e.m.f. is zero or less than the cut-out value, the respective semiconductor circuit element connected to the energised winding is turned on. On starting of the motor from its stopped position the occurrence of an unwanted cut-out is in this way prevented.
Furthermore it is advantageous according to the invention if, in the starting phase, a starting current limitation circuit is operable which, up to a predetermined level of the starting current, turns the semiconductor circuit elements on and off until the counter-e.m.f. produced in the motor windings has reached such a level that only a limited motor current can flow. Thereby the circuit takes account of the fact that the counter-e.m.f. offsets the operating voltage such that automatic limitation of the current flowing through the windings ensues.